My invention relates to a control circuit for phase locked loops, and particularly to a bandwidth control circuit for phase locked loops used in a frequency synthesizer.
Phase locked loops are used in frequency synthesizers to provide a relatively wide range of selectable frequencies having the stability of a reference oscillator which is typically crystal controlled. Such a wide frequency range requires that the filter in the phase locked loop have a variable bandwidth so that when a frequency change is programmed or required, such a change can be made in a small amount of time. The rate of change depends on the loop control voltage, which in turn is limited first by the amount of loop gain available. Second, the loop control voltage is further limited when a relatively low cutoff frequency filter is inserted into the phase locked loop control line to reduce noise and frequency modulation of the voltage controlled oscillator by the reference frequency. In prior art circuits of which I am aware, most filter switching in a phase locked loop has been accomplished by using a fixed timing circuit (such as a one shot multivibrator) which initially allows locking with a relatively high filter bandwidth. After the timing circuit times out, it switches the filter to a relatively low bandwidth. However, there are several problems associated with such a timing circuit. First, if for some reason the voltage controlled oscillator goes out of lock because of a noise burst or pulse from the power supply voltage, the normal lock detector might not detect the loss of lock for many milliseconds, so that a transmitter whose frequency source is the voltage controlled oscillator might produce a carrier or high level noise over a good portion of the adjacent frequency spectrum. The second problem is that the fixed time allowed for the voltage controlled oscillator to settle to the new commanded frequency may not be long enough for the particular frequency change involved. When the bandwidth is reduced after this fixed time but before the transmitter has settled to the commanded frequency, the transmitter may produce undesired frequencies with associated interference until its frequency is correct.
From an ideal standpoint, relatively fast and wide frequency changes should be made with a relatively wide bandwidth filter circuit in the phase locked loop. After the voltage controlled oscillator produces the new commanded frequency, the filter circuit should provide a relatively narrow bandwidth.